Development of a photoelectric conversion device in which a thin film transistor (TFT) and a PIN diode (PIN type diode) are combined has been in progress. As the PIN diode that functions as an optical sensor, a PIN diode, in which three layers of P-type amorphous silicon that is doped with boron (element symbol: B) as an impurity, high-resistance amorphous silicon that is not doped with an impurity, and N-type amorphous silicon that is doped with phosphorous (element symbol: P) as an impurity are stacked, has been used. Band-gap energy of the amorphous silicon, which constitutes the PIN diode, is approximately 1.6 eV, and is sufficiently smaller than optical energy in a visible light region (wavelength: 400 nm (3.2 eV) to 700 nm (1.8 eV)). Accordingly, the optical sensor that uses the PIN diode absorbs almost all of light beams with wavelengths in the visible light region, and generates a carrier. As a result, the optical sensor functions as a photoelectric conversion element with efficiency.
Recently, a high-performance photoelectric conversion device, in which an oxide semiconductor TFT with high electric field effect mobility, and a PIN diode are combined, has been actively developed. When using the oxide semiconductor TFT as a switch or an amplification circuit, it is possible to extract a light intensity distribution after conversion into an electrical signal with excellent reproducibility, and it is possible to reduce an area occupied by the amplification circuit.
As described above, there are disclosed various technologies relating to the photoelectric conversion device in which the switch that uses the oxide semiconductor TFT, and the optical sensor that includes the PIN diode or the amorphous silicon TFT are combined.
For example, WO2011/135920 (hereinafter, referred to as Patent Document 1) discloses a photoelectric conversion device in which a switch constituted by a top gate-type oxide semiconductor TFT with a high ON/OFF ratio, and an optical sensor constituted by a bottom gate-type amorphous silicon TFT with high light sensitivity are combined. In a technology disclosed in Patent Document 1, it is possible to connect a gate line and a drain line without forming a contact hole due to the combination of the top gate type and the bottom gate type, and thus it is possible to reduce a photolithography process.
In addition, Japanese Patent Application Publication No. 2010-153834 (hereinafter, referred to as Patent Document 2) discloses a high-performance photoelectric conversion device in which an oxide semiconductor TFT (that constitutes a switch and an amplification circuit) with high electric field effect mobility, and a PIN diode are combined. In a technology disclosed in Patent Document 2, the oxide semiconductor TFT is used, and thus it is possible to extract a light intensity distribution after conversion into an electric signal with excellent reproducibility. In addition, it is possible to reduce an area occupied by the amplification circuit.
In addition, Japanese Patent Application Publication No. 2006-165530 (hereinafter, referred to as Patent Document 3) discloses a technology in which a pigment is adsorbed to an oxide semiconductor in order for the oxide semiconductor to have visible light sensitivity, and the oxide semiconductor is used in an optical sensor or an X-ray sensor as a photodiode of two-terminal element.
In addition, in Japanese Patent Application Publication No. H5-235398 (hereinafter, referred to as Patent Document 4), with regard to an optical sensor that is constituted by amorphous silicon TFT, a structure, in which a top gate electrode formed of a transparent material is added, is disclosed. In a technology disclosed in Patent Document 4, a potential of the top gate electrode is fixed to a potential that is lower than a potential of a source electrode, and a threshold voltage is shifted to a positive direction so as to reduce a dark current when a gate voltage is 0 V. According to this, a high bright-dark current ratio is realized.
In addition, in “Oxide based Photosensor Thin Film Transistor for Interactive Display” by Seung-eon Ahn and seven persons, The Proceedings of AM-FPD 2013, The Japan Society of Applied Physics, July 2013, p. 67-70 (hereinafter, referred to as Non-Patent Document 1), a technology, in which the oxide semiconductor TFT is used in both a switch and an optical sensor, is disclosed. In addition, in Non-Patent Document 1, a shied for light shielding is provided on an upper portion of the oxide semiconductor TFT for the switch, but the shield is not provided on an upper portion of the oxide semiconductor TFT for the optical sensor. In a technology disclosed in Non-Patent Document 1, optical sensing is performed by using a phenomenon in which an off-current in the oxide semiconductor TFT for the optical sensor increases in the case of performing irradiation with light beams of green to bluish-purple, which have a wavelength of 550 nm or less.